Improve repmgr configuration file handling

Previous behaviour was somewhat counterintuitive, with an error
message being logged if no configuration file provided or found,
even though this is not actually an error.

Configuration files now handled like this:

- if a configuration file is explicitly provided (-f), error out
  if not found.

- if no configuration file explicitly provided, attempt to open
  default configuration file; if this does not exist, log
  notice and continue with default values.

Also, for 9.4 and later add a hint about replication slot usage
if 'use_replication_slots' not set.

config.c

@@ -65,8 +65,9 @@ parse_config(const char *config_file, t_configuration_options * options)
 	 */
 	if (fp == NULL)
 	{
-		log_err(_("Did not find the configuration file '%s', continuing\n"),
-				config_file);
+		log_notice(_("No configuration file provided and default file '%s' not found - "
+					 "continuing with default values\n"),
+				   config_file);
 		return;
 	}